Typically, eval is used in shell scripts,
args is a line of code that contains shell variables.
eval forces variable expansion to happen first
and then runs the resulting command.
This "double-scanning" is useful any time shell variables
contain input/output redirection symbols, aliases, or other shell variables.
(For example, redirection normally happens before variable
expansion, so a variable containing redirection symbols must be
expanded first using eval; otherwise, the redirection symbols
remain uninterpreted.) See the C-shell eval (Section 5)
for another example.
This fragment of a Bourne shell script shows how eval constructs a command that is interpreted in the right order:
for option do case "$option" in #define where output goes save) out=' > $newfile' ;; show) out=' | more' ;; esac done eval sort $file $out